This invention relates generally to Exothermic Reactions and methods, and more particularly to apparatus and methods for initiating self propagating exothermic reactions.
Examples of self propagating exothermic reactions are found in the CADWELD(copyright) process and the Thermit(copyright) process. CADWELD(copyright) is a trademark of Erico International, Inc., Solon, Ohio, U.S.A., and Thermit(copyright) is a trademark of Th. Goldschmidt AG, Essex, Germany. Exothermic mixtures are basically a combination of a reductant metal and usually a transition metal oxide. An example is aluminum and copper oxide which upon ignition supplies enough heat to propagate and sustain a reaction within the mixture. It is usually the molten metal product or the heat of this reaction which is then used to produce a desired result. The CADWELD(copyright) process produces, for example, a mixture of molten copper and aluminum oxide or slag. The molten copper has a higher density than the slag and is usually directed by a mold to join or weld copper to copper or steel to steel. The aluminum oxide slag is removed from the weld or joint and discarded. Another common mixture is iron oxide and aluminum. Where only the heat of the reaction is used, the heat may be used to fuze brazing material, for example.
Mixtures of this type do not react spontaneously and need a method of initiating the reaction. This initiation method involves generating enough localized energy to enable the reaction to begin. Once the reaction has begun it becomes self sustaining and requires no further energy to proceed to completion. There are numerous combinations of reductant metals and transition metal oxides that can react exothermically. These reactions and the energy required to initiate these reactions vary greatly depending on the properties of the reactants and the localized conditions. Two common mixtures are combinations of copper oxide and aluminum, and iron oxide and aluminum. This invention is concerned with the initiation of exothermic reactions, and apparatus or packaging for utilizing such reactions.
Currently the most common way of making welds or joints with the CADWELD(copyright) process involves the use of split graphite molds. The conductors or items to be joined are thoroughly cleaned and then placed in the appropriate location to project into a weld chamber in the graphite mold. The molds may include a crucible above the weld chamber connected to the weld chamber by a tap hole. The mold is then securely closed and locked usually with a toggle clamp, and a metal disk is positioned in the crucible over the tap hole. An appropriate amount of exothermic material is emptied into the crucible on top of the disk, and the traditional starting powder or material, which is essentially a much finer exothermic material, is sprinkled over the top of the welding material. The mold cover is then closed and the reaction initiated through the use of a flint ignitor.
The starting powder or material sprinkled on top of the exothermic material has a lower ignition temperature and is easily ignited by the flint gun while the flint gun cannot normally ignite the exothermic material directly. When the exothermic material is ignited, the molten metal phase separates from the slag and melts through the metal disk. The molten metal then is directed via the tap hole to the weld chamber and the conductors to be joined. Once the metal has solidified the mold is opened and the slag is separated from the weld joint. The mold is cleaned and readied for reuse for the next connection.
Because of the starting powder""s low ignition temperature and deficiencies in handling and shipping, much effort has been made to find a reliable and low cost alternative ignition system for the exothermic material. A number of electrical systems have been devised which range from simple spark gaps to bridge wires or foils, to much more esoteric devices such as rocket ignitors. Such efforts are seen, for example, in prior U.S. Pat. Nos. 4,881,677, 4,879,952, 4,885,452, 4,889,324 and 5,145,106. For a variety of reasons, but primarily because of power requirements, dependability, and cost, such devices have not succeeded in replacing the standard starting powder/flint gun form of initiating the self propagating exothermic reactions.
The packaging of the system is also important. Many such applications are performed outdoors or in the field, and portability and ease of use is important. For example, no one wants to carry around a car battery to use as an ignition system. Also, the system should be capable of use with few light weight, easily used and cleaned or preferably disposable components.
An ignition system for such materials should be able to produce about thirty (30) joules of energy. The system should require no special shipment classification or labeling which is now required in some jurisdictions with certain starting powders or materials. The system components should be user friendly to the point of being easily carried and operable with only one hand. The system should be readily transportable to difficult working ares or confined or cramped spaces. The system components should weigh less than 1.5 kgs or about the weight of a partially full attache case. As important, the ignition must be reliable and repeatable without requiring frequent new batteries or a frequent recharge, and must be economic to make and use.
It would also be advantageous if the ignition system did not use wires or projecting wires to which a power source needs to be connected. Power may be connected to such wires by allegator clips, for example, one for each wire. The wires tend to get bent, dislodge, catch on things, or broken off and may easily short. Many times lack of reliability is simply a faulty clip or clip connection, and this usually requires some probing or testing or repositioning of the clips or wires before the problem is even located. It would be desirable if no wires were used and a proper connection could be made with a single clip.
The exothermic materials reaction initiation system includes a power source for producing a voltage spike or surge and an ignitor comprising two strips of metal foil separated by a layer of insulation. The ignitor includes one or more strategically shaped and placed distortions in the form of one or more punched holes. It has been discovered that a relatively low voltage spike applied to the foil strips will create a spark plasma of sufficient shape and projection to ignite substantially adjacent exothermic mixture and have the reaction propagate to completion. The hole is formed by a small conical tip punch. The conical tip shapes the hole like a horn and the through opening at the small end of the horn acts as a vent.
The power source may be a battery powered capacitor discharge unit which will create the voltage to be delivered to the metal foil strips by two electrodes forming the opposite gripping surfaces of a spring clothespin type clip which is simply clamped on a projecting end of the metal foil strip with one foil strip in contact with one electrode. This maintains the required polarization. The battery or batteries may be relatively small, easily replaced, and rechargeable. The capacitor discharge unit need only create a voltage for about thirty (30) joules for most applications, and this is sufficient to create the shaped and vented spark plasma at the disruption or hole.
The ignitor unit is preferably thin strips of conductive foil which are separated by a thin layer of paper insulation adhered to the foil with thin layers of adhesive so that the layers do not readily separate and no air gaps or bubbles occur. Preferably the insulation extends beyond the foil, particularly at the ends so that no spark-overs occur at the edges or at a location other than at the strategically positioned disruption. The adhesive may be conductive to enhance current flow through the metal foil. The entire assembly including the conductive foil, the adhesive and the insulation may be only a few mils in thickness, and the strip assembly may be bent, folded or even twisted to some degree without damage to its integrity and purpose.
The selection of the metal or type of foil depends to some degree on the application. Since the shaped and vented plasma creating spark discharge at the disruption involves some controlled metal splatter and fusion of the foil, and since the ignitor in some applications will be consumed or partially consumed by the reaction, it is important that the metal of the foil be compatible with the reaction. Thus for copper welding, copper would be a suitable foil material. The paper and adhesive are also selected so as not to contaminate the exothermic process.
Although the ignitor assembly has many applications in connection with reusable crucibles and molds designed to contain and direct the reaction and the products of the reaction, a preferred form is built into a self contained and expendable package of exothermic material. The package may be positioned on a reusable or expendable mold, and the ignitor assembly may be unfolded or bent to extend or project from the package. The ignitor is connected to the power source, and the reaction initiated. A disk in the bottom of the package is fused by the reaction, and the molten metal formed runs into a reusable or expendable mold. The power source is then disconnected and the package discarded when cooled.
In some embodiments, the ignitor may include two or more holes or spark plasma creating disruptions. If the holes are of the same size, they will fire essentially concurrently. Such ignitors may be used for large volumes of exothermic materials to obtain multiple point ignition. If the holes vary in size, sequential ignition can be obtained. Multiple hole ignitors may also be used for larger charges or redundancy.
In addition to the ignitor and methods of making the ignitor, the invention also relates to the methods of ignition disclosed, as well as the applications and packages for utilizing the ignitor, all of which economically eliminate the use of the starting powder and materials, and the flint gun ignitor.
To the accomplishment of the foregoing and related ends, the invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.